In recent years, water-absorbent resins have been widely used in the fields of hygienic materials such as disposable diapers, sanitary napkins and incontinence pads.
For water-absorbent resins as described above, cross-linked products of partially neutralized polymers of acrylic acid are preferred because they have many advantages, including the followings: they have better water-absorption performance; their raw materials such as acrylic acid has easy industrial availability, and therefore they can be produced with stable quality and low cost; and they show no shortcomings such as in which decomposition and degradation are likely to occur.
On the other hand, an absorbent article such as a disposable diaper, a sanitary napkin or an incontinence pad is formed with: an absorbent material which is mainly arranged in a center portion and which absorbs and holds body fluids such as urine and menstrual blood that are excreted from the body; a front surface sheet (top sheet) which is arranged on a side in contact with the body and which is liquid permeable; and a back surface sheet (back sheet) which is arranged on a side opposite to the side in contact with the body and which is not is liquid permeable. The absorbent material is formed with a hydrophilic fiber such as pulp and a water-absorbent resin.
In recent years, in terms of the design, the convenience at the time of carrying, the efficiency at the time of distribution and the like, it has been more required to reduce the thickness and the weight of the absorbent article. Furthermore, in terms of environmental protection, there are growing needs on a so-called eco-friendly orientation in which resources are effectively utilized to minimizing the usage of natural materials such as trees that it takes a long period of time to grow. Conventionally, as a method for reducing the thickness of the absorbent article which is generally performed, for example, there is a method of reducing a hydrophilic fiber such as the crushed pulp of wood, which serves to fix a water-absorbent resin in an absorbent material, and of increasing the water-absorbent resin.
The absorbent material in which the ratio of the hydrophilic fiber is lowered and in which a large amount of water-absorbent resin is used is preferable for reducing the thickness in terms of reducing the bulky hydrophilic fiber and holding a liquid. However, for example, in a case that a load is applied by deformation, pressure or the like to the absorbent material containing the water-absorbent resin, for example, when an infant wearing an absorbent article whose thickness is reduced sits, the re-wet (liquid back) of a to-be-absorbed liquid may not be able to be fully prevented. Furthermore, in the case of such an absorbent article, the absorbent article cannot stand urination which is performed a plurality of times, and thus a user may have an uncomfortable feeling.
A large amount of water-absorbent resin is formed into the shape of a soft gel by the absorption of the liquid, and furthermore, when a load is applied to the gel, a so-called “gel blocking phenomenon” occurs, and thus the liquid diffusibility is significantly lowered, with the result that the permeation rate of the liquid by the absorbent material may be lowered. The “gel blocking phenomenon” refers to a phenomenon in which especially when the absorbent material where a large amount of water-absorbent resin is densely present absorbs the liquid, the water-absorbent resin present around a front layer absorbs the liquid to form a soft gel around the front layer and the gel becomes dense to prevent the permeation of the liquid into the absorbent material, with the result that the water-absorbent resin therewithin cannot efficiently absorb the liquid.
Hence, as a means for preventing the problem occurring when the hydrophilic fiber is reduced and a large amount of water-absorbent resin is used, for example, the following methods have so far been proposed: a method (see Patent Document 1) of using a hydrogel water-absorbent polymer having specific saline flow conductivity, performance under pressure and the like; and a method (see Patent Document 2) of using a water-absorbent resin obtained by heating and processing a specific surface crosslinking agent on a specific water-absorbent resin precursor.
In these methods, however, the absorption performance of the absorbent material where a large amount of water-absorbent resin is used is not always satisfied, and there is a tendency that a to-be-absorbed liquid cannot be captured, and that thus the liquid disadvantageously leaks.